Motion of an isolated liquid plug inside a capillary tube: effect of contact angle hysteresis

Dynamics of a single, small and isolated partially wetting liquid plug (of known length L and wettability), placed at rest inside a long, dry, circular capillary tube (D = 1.5 mm), and subsequently quasi-statically pushed from one end by applying air pressure, the other end being kept exposed to atmosphere, are reported. The air pressure first overcomes the ‘static’ friction manifested by the three-phase contact line at the advancing and receding menisci, and then, the plug motion gets initiated, eventually leading to a terminal velocity (Ca ~ 2.8 × 10⁻⁵), when pressure force balances net frictional resistance due to viscous and surface forces. It is seen that, under steady motion, the curvature profiles of the advancing and receding menisci of liquid plug, respectively, remain the same, independent of the plug length. Steady-state pressure drop is dominated by the contribution due to contact angle hysteresis, which is also independent of the plug length. Increasing the system wettability drastically decreased the contact angle hysteresis and the associated net pressure drop.